A mobile-friendly website is one that is designed to work the exact same way across devices. This means that nothing changes or is unusable on a computer or mobile device. Features like navigation drop-downs are limited, as they can be difficult to use on mobile. And no Flash animation is used. The website is literally the same across the board, with no usability concerns regardless of the device on which it is being viewed.

A responsive website is one that responds (or changes) based on the needs of the users and the device (mobile device in this example) that they're viewing it on. Here, text and images change from a three-column layout to a single column display. Unnecessary images are hidden so they don’t interfere or compete with the more important information on the site's smaller display.

If you are on a computer, you can tell if a site is responsive by reducing your browser’s window size from full screen down to very small. If the appearance of the text, images and menu change as you get smaller, the site is responsive.

Key features of responsive websites:

Dynamic content that changesNavigation is condensedOptimized imagesCorrect padding and spacingReliant on mobile operating systems to function